Devices such as a touch pad and a touch panel equipped with a sensor for detecting a contact position of an object such as a finger and a pen are widely used as input interfaces for a smartphone and the like. There are various types of sensors for detecting a contact position of an object, such as of a resistive-film type and of an optical-type. Especially, because a capacitive sensor is relatively simple and compact, many mobile devices have been adopting the capacitive sensor as their input interfaces in recent years.
There are multiple types of capacitive sensors. Major types of capacitive sensors are a self-capacitive sensor and a mutual capacitive sensor. The self-capacitive sensor detects a change in capacitance (self-capacitance) between a sensing electrode and an object (ground). Therefore, in order to detect self-capacitance at multiple locations, the same number of sensing electrodes as the locations is required. In contrast, because the mutual capacitive sensor detects a change in capacitance (mutual capacitance) between a driving electrode and a sensing electrode caused by an approach of an object, the mutual capacitive sensor can detect a change in capacitance at multiple locations with a single sensing electrode. Accordingly, the mutual capacitive sensor is more suitable for multi-point sensing than a self-capacitive sensor.
In the mutual capacitive sensor, when a finger approaches a portion at which mutual capacitance is formed (an intersection between a driving electrode and a sensing electrode), the mutual capacitance is reduced. In general, mutual capacitance formed between a driving electrode and a sensing electrode is very small, and a change of the mutual capacitance is even smaller. Thus, the mutual capacitance is subject to change depending on temperature and the like. Accordingly, in a general mutual capacitive sensor, a reference value for determining a change in mutual capacitance is appropriately updated at a predetermined time. Specifically, when a state in which a mutual capacitive sensor is not touched by an object such as a finger is continued, mutual capacitance is detected and magnitude of the detected mutual capacitance is set as a new reference value. Thereafter, a value obtained by subtracting detected mutual capacitance from the reference value is used as data indicating a change in mutual capacitance.
When an object (such as a human body), capacitively coupled with a ground strongly, approaches a capacitive sensor, its mutual capacitance decreases. However, when a conductor capacitively coupled with a ground weakly, such as a coin, approaches the capacitive sensor, its mutual capacitance increases. Although only occurrence of an increase of mutual capacitance is not regarded as approach of an object, if the above-described update of a reference value is performed while an object such as a coin is close to a capacitive sensor, and if mutual capacitance is reduced by the object being removed, the reduction of the mutual capacitance may be erroneously detected as an approach of an object.
Patent Document 1 describes a method of quickly resetting a reference value that causes such a false detection. In a device described in Patent Document 1, when an amount of change in mutual capacitance becomes equal to or exceeds a predetermined threshold, a cumulative value of the amount of change is stored in a storage unit, and a state transits to a floating conductor monitoring state for monitoring a floating conductor of floating potential. When an absolute value of a difference between an amount of change in mutual capacitance and the reference value is substantially equal to an absolute value of a difference between the cumulative value stored in the storage unit and the reference value, and a sign of these two differences are opposite, mutual capacitance is measured again and the measured mutual capacitance is set again as a new reference value. If, while the device is in the floating conductor monitoring state, the device recognizes a finger and thereafter the device transits to a state in which the finger is not recognized, it is presumed that there is no conductor such as a coin, and thus the floating conductor monitoring state is canceled.